/Engine/Source/Runtime/Engine/Classes/Kismet/KismetMathLibrary.h
Given a direction vector and a surface normal, returns the vector reflected across the surface normal. Produces a result like shining a laser at a mirror! The W element is ignored.
Reflected vector.
static FVector4 Vector4_MirrorByVector3 ( const FVector4 & Direction, const FVector4 & SurfaceNormal )